credential: sanitize the user prompt

When asking the user interactively for credentials, we want to avoid misleading them e.g. via control sequences that pretend that the URL targets a trusted host when it does not. While Git learned, over the course of the preceding commits, to disallow URLs containing URL-encoded control characters by default, credential helpers are still allowed to specify values very freely (apart from Line Feed and NUL characters, anything is allowed), and this would allow, say, a username containing control characters to be specified that would then be displayed in the interactive terminal prompt asking the user for the password, potentially sending those control characters directly to the terminal. This is undesirable because control characters can be used to mislead users to divulge secret information to untrusted sites. To prevent such an attack vector, let's add a `git_prompt()` that forces the displayed text to be sanitized, i.e. displaying question marks instead of control characters. Note: While this commit's diff changes a lot of `user@host` strings to `user%40host`, which may look suspicious on the surface, there is a good reason for that: this string specifies a user name, not a <username>@<hostname> combination! In the context of t5541, the actual combination looks like this: `user%40@127.0.0.1:5541`. Therefore, these string replacements document a net improvement introduced by this commit, as `user@host@127.0.0.1` could have left readers wondering where the user name ends and where the host name begins. Hinted-at-by: Jeff King <peff@peff.net> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>